1. Field of the Invention
The invention relates to a mixing device for mixing a low-viscosity fluid into a high-viscosity fluid. The high-viscosity fluids envisaged are primarily polymer melts; the low-viscosity fluids are primarily additives that are to be incorporated into polymer melts. The additive can be both soluble and insoluble in the melt. The difference in the viscosities of the fluids is generally very large; the viscosities can differ by a factor of two or more orders of magnitude. Both liquids and gases can be used as low-viscosity fluids. The high-viscosity fluids can contain solids. The invention also relates to processes that can be performed with the device.
2. Description of the Prior Art
A method for mixing liquids with greatly differing viscosities is known from DE-PS 23 15 114. With this method, the low-viscosity fluid is pre-mixed with a branch flow of the high-viscosity fluid using a dynamic mixer. The resulting pre-mixture is combined with the non-branched off part of the flow of the high-viscosity fluid and the entire flow is then mixed further in a static mixer. The known process can be used to advantage when incorporating low-viscosity additives into polymer melts, since it is possible for the additives to be mixed in homogeneously and gently, i.e. without severe shear stressing of the polymer. According to the teaching of DE-PS 23 15 114, the results obtained from mixing processes using only gentle-action static mixers are very unsatisfactory.
The use of dynamic mixers is, however, very costly and maintenance-intensive; and some degradation of the polymers has to be accepted in such mixers owing to the shearing and warming due to the stirring motion. The problem underlying the invention is thus to devise a mixing device which manages without dynamic mixers and permits homogeneous mixing of low-viscosity additives into polymer melts.